The present invention relates to a procedure for characterizing a spray composed of spherical particles, by determining information related to its number density (particles per unit volume) in points located within a measurement plane intersected by the spray. The characterization is based on the scattering of a laser light beam by the particles contained in the spray, when the particle size is large enough for the Mie scattering theory to apply. The invention includes also the apparatus to materialize the procedure.
There are a number of techniques which, working in the Mie regime, exploit the scattering of laser light by a spray of particles to extract information related to the spray number density.
A technique known as laser diffraction particle sizing determines the spatial number size distribution of the spray material intersected by a laser beam by measuring the forward scattering pattern through a set of photodetectors located in the focal plane of a receiving lens. The application of this technique for characterizing sprays is used in the apparatus developed by the British company Malvern Instruments, where the photodetectors are ring-shaped photodiodes. Diffraction is the dominant scattering mode in the collection angles sensed by the photodiodes. Since the far field diffraction pattern for spherical particles is a well known function of the particle size and the laser wavelength, the technique obtains the size distribution by matching the measured diffraction pattern to that obtained from a collection of particles distributed in a finite number of size classes. In addition, the attenuated laser intensity is measured with a photodiode located in the receiving lens axis. Knowing the attenuation of the laser beam and the spray size distribution allows the technique to infer the spray number density. Since the technique is an on-axis method, the laser diffraction technique can only give a measure of the spray number density averaged along the volume intercepted by the spray and the laser beam.
Another family of methods developed to obtain information related to the spray number density is based on counting techniques initially used for fluid anemometry purposes. The optical arrangement is similar to that of the dual beam laser Doppler velocimetry system (LDV). The interference pattern formed at the intersection of two coherent laser beams modulates the light scattered by particles passing through the probe volume. A set of photodetectors is placed to detect the off-axis particle scattering activity. The velocity of the particles can be obtained by recording the Doppler frequency of the scattering light produced by the particles intersecting the probe volume. The size of the particles can be inferred either from a measurement of the visibility of the Doppler signal (technique disclosed in U.S. Pat. No. 4,329,054), or from the phase difference of the Doppler signal detected in spatially separated photodetectors (technique disclosed, under different hardware implementations, in International Patent WO 84/04592 and in U.S. Pat. No. 4,540,283). A critical aspect of the obtained size distribution is that it is biased towards the large particle sizes, since the laser beams have a gaussian intensity decay from their axis, and the large particles scatter more light intensity than the smaller ones. As a consequence, the characteristic probe volume of the large particles is greater than that of the smaller ones. A deconvolution based on the statistics of the transit time and velocity of the particles can be applied to eliminate this bias. The velocity of the particles also biases the number-size distribution but, since this velocity is known, a transformation can be introduced to obtain the unbiased spatial number-size distribution. The number density information can be obtained by knowing the cross-stream area of the formed probe volume and the number of particles passing through it during a known measurement time. Being based on off-axis scattering analysis, these counting techniques can perform highly spatially resolved number density measurement. Determination of the probe volume cross-stream area and of the effective measurement time encounters, however, severe difficulties when characterizing sprays subjected to high optical attenuation levels and/or flowing in directions other than that perpendicular to the laser beams bisector.
The invention disclosed hereinafter performs spatially resolved measurements of a quantity proportional to the spray number density in a measurement plane intersected by the spray and characterized by arbitrarily large optical attenuation levels, and arbitrary spray trajectory angles. In addition, the technique of the invention can be combined with independently obtained measurement of the spray number-size histogram and of the velocity-size correlation to explicitly infer the spray number density and volume flux without knowledge of the probe volume cross-stream area or of the effective measurement time.